1. Field of the Invention
The present invention relates to an inverter circuit for use in an induction-heating cooking apparatus and a method for operating the same, which block an inverter circuit from being operated under a resonance frequency according to a substance of a heating container upon receiving a low-voltage signal in a high-output state of the induction-heating cooking apparatus, prevent a switch having a relatively low current from being damaged, resulting in increased endurance.
2. Description of the Related Art
Generally, a cooking appliance (also called a cooking apparatus) includes: a main body having a control board capable of determining whether a power-supply signal is received upon receiving a command signal from a user; a cooking container seated in the main body, for including food therein; and a cooking heater installed to a lower part of the cooking container or an inner side of the main body to cook the food included in the cooking container.
An induction-heating scheme arranges coils in the main body wherein the cooking container is seated at intervals of a predetermined distance, and allows an eddy current to be generated in the cooking container formed of a magnetic material due to a magnetic field generated when a current signal flows in the coil, thereby heating the cooking container. A variety of kitchen appliances, for example, a rice cooker, a cook-top range, and an electric pan, etc., have been designed to use the above induction-heating scheme.
An inverter circuit for use in the above-mentioned induction-heating cooking apparatuses switches on or off a switch formed of an IGBT (Insulated Gate Bipolar Transistor), applies a high-frequency current having high power to the coil, and heats the container located on the coil.
The inverter circuit for use in the conventional induction-heating cooking apparatus will hereinafter be described with reference to FIG. 1. Referring to FIG. 1, the inverter circuit includes an AC power-supply unit 1 for generating a common AC power-supply signal; a rectifier 2 for rectifying the AC power-supply signal; a filter unit 3 for filtering a power-supply signal rectified by the rectifier 12; and an inverter unit 4 for receiving the filtered power-supply signal from the filter unit 3, switching on the switch, and providing the coil with a high-output power-supply signal.
An input voltage detector 5 is connected to the AC power-supply unit 1, and detects a voltage applied to the inverter circuit. An input voltage compensator 6 compensates for an output control signal generated by a microprocessor of a cooking apparatus according to a variation of the detected input voltage.
In other words, if the input voltage detector 5 detects an input voltage higher than a reference rated input voltage, the input voltage compensator 6 reduces a voltage value of an inverter output control signal generated from a microprocessor. Otherwise, if the input voltage detector 5 detects an input voltage less than the reference rated input voltage, the input voltage compensator 6 increases a voltage value of the inverter output control signal in such a way that it compensates for an inverter output control signal according to a variation of the input voltage.
The output controller 7 generates a frequency control signal capable of controlling an operation frequency of the inverter unit 4 according to an output voltage level generated from the input voltage compensator 6, and generates a constant output signal irrespective of the variation of the input voltage.
In more detail, the output controller 7 generates a frequency control signal, such that it increases the operation frequency when the input voltage is higher than a reference output control signal, and reduces the operation frequency upon receiving a voltage signal less than the reference output control signal.
Upon receiving the frequency control signal, a pulse generator 8 generates a driving pulse to allow the switch of the inverter unit 4 to be switched on or off at the operation frequency. A switch driver 9 transmits the driving pulse to a gate of the switch, and switches on the switch, so that it generates a constant-output signal.
In this case, the operation frequency of the inverter unit 4 is controlled by the output controller 7. The degree of magnetism is changed according to a substance of a cooking container seated on the coil, and a resonance frequency is also changed due to the changed magnetism.
Therefore, the output controller 7 establishes an operation frequency to prevent the inverter unit 4 from being operated under the resonance frequency caused by the substance of the cooking container, such that it increases power output efficiency, and drives the inverter according to a ZVS (Zero Voltage Switching) scheme.
However, when the cooking container is installed in the conventional induction-heating cooking apparatus, or a low-input voltage is applied to the induction-heating cooking apparatus, a resonance frequency is different from that determined by the microprocessor, such that it is difficult to guarantee the ZVS operation of the inverter, as shown in FIG. 2.
Referring to FIG. 2, if a resonance frequency f2 of the cooking container formed of a substance B is set to an operation limitation frequency of the inverter, the inverter can escape from the ZVS operation area when another cooking container formed of a substance A having a resonance frequency f1 is seated, such that the cooking container is unable to generate the maximum output level.
Upon receiving an input voltage less than a rated input voltage when the cooking container formed of the substance B is operated at the resonance frequency f2 capable of generating the maximum power signal P2, the input voltage compensator 6 increases an inverter output control signal, and the output controller 7 generates a frequency output control signal to reduce the switching operation frequency, such that the operation of the inverter escapes from a predetermined area ZVS2.
Therefore, if the inverter operation escapes from the ZVS2 operation area, the switch encounters an excessive switching operation and a high instantaneous current when it is switched on, such that the IGBT switch may be damaged. As a result, there arises a malfunction of the induction-heating cooking apparatus, resulting in unnecessary repair costs and deterioration of endurance.